Change the value of middlemouse.paste to true. This will let you paste a clipboard content to any text field with the middle mouse button.

Using this tweak coupled with our beloved AutoCopy extension, you could select text on-page and paste it into a textarea (like in a comments response) with a simple click, drag, and mousewheel hit. (Update: Oh, duh. If you've already got AutoCopy installed, mousewheel paste is already an extension option.) See more of our favorite Firefox 2 about:config tweaks.